Electrodes are widely used to store electrical energy, including primary (non-rechargeable) battery cells, secondary battery cells, fuel cells, and capacitors. Because of various competing performance criteria that need to be considered when designing electrodes, many electrodes are built using two or even more parts with different constituent materials. For example, an electrode can be constructed using a film of active electrode material backed by a current collector. Such electrodes are often used in double layer capacitors, which are also known as electrochemical capacitors, supercapacitors, and ultracapacitors.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for this volumetric and weight efficiency. First, the width of the charge separation layers is very small, on the order of nanometers. Second, the electrodes can be made from a porous material, having very large effective area per unit volume, i.e., very large normalized effective surface area. Because capacitance is directly proportional to the electrode area, and inversely proportional to the width of the charge separation layer, the combined effect of the narrow charge separation layer and large surface area results in capacitance that is very high in comparison to that of conventional capacitors with similar physical measurements. High capacitance enables double layer capacitors to receive, store, and release large supplies of electrical energy.
Another important performance parameter of a capacitor is its internal resistance, also known as equivalent series resistance (ESR). Internal resistance limits both charge and discharge rates of a capacitor, because the resistance curtails the current that flows into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications. In automotive applications, for example, a capacitor used as the energy storage element powering a vehicle's engine has to be able to provide high instantaneous power during acceleration, and to receive bursts of power produced by regenerative braking. In internal combustion vehicles, a capacitor can be used to power a vehicle's starter, requiring high power output in relation to the size of the capacitor.
Internal resistance also creates heat during both charge and discharge cycles. Heat causes mechanical stresses and speeds up various chemical reactions, thereby accelerating capacitor aging. Moreover, the energy converted into heat is lost, decreasing the efficiency of the capacitor. It is therefore desirable to reduce internal resistance of capacitors.
Active materials used for electrode construction—activated carbon, for example—usually have rather limited specific conductance. Thus, large contact area may be desired or required to minimize the contact resistance between the electrode's active material and the electrode's terminal. The active material may also be too brittle or otherwise unsuitable for directly connecting to electrical terminals. Additionally, the active electrode material may have a relatively low tensile strength, necessitating the use of a mechanical support element in some applications. For these reasons, electrodes often incorporate current collectors.
A current collector is typically a sheet of conductive material on which the active electrode material is deposited. Aluminum foil is commonly used as the current collector material of an electrode. In one electrode fabrication process, for example, a film that includes activated carbon powder (the active electrode material) is produced, and then attached to a thin aluminum foil using an adhesive binder layer. To improve the quality of the interfacial bond between the film of active electrode material and the current collector, the combination of the film and the current collector is processed in a pressure laminator, for example, a calender or another nip. Presence of the binder layer and the process of high pressure lamination increase the bonding forces between the film and the current collector, and reduce the equivalent series resistance of the resulting energy storage device.
In known electrode fabrication processes, the adhesive binder is generally applied to the collector prior to lamination as a wet coating. The coated current collector and the active electrode film are then brought into contact with each other (i.e., stacked) and laminated. Next, the electrode is dried to eliminate volatile solvents present in the wet adhesive. Because the current collector typically has active electrode films attached on both sides, the lamination process may need to be repeated to attach the second film. Alternatively, the binder can be applied to both sides of the current collector, and the collector and the two active electrode films can be laminated together in one step.
During the lamination process, it is important to achieve good registration between each adhesive binder layer and the corresponding active electrode film, and it is also important to achieve good registration between the two active electrode layers. (In the present context, registration means alignment or physical coincidence in the plane of the current collector.) There are several reasons for having good registration. For example, the bond between the current collector and the film may be weakened if binder is not present between some area of the film and the current collector. Additionally, the interfacial resistance will also increase. By way of another example, some area of the current collector may need to remain accessible for attachment of a terminal or connection to another electrode. To ensure that such clear area exists, the current collector can be made slightly wider than the active electrode film. The difference in the respective widths exceeds the maximum registration error, so that the clear area is available at the required location. Unfortunately, it can be difficult to achieve good registration between the active electrode films and the adhesive binder layers coated on the current collector. It can also be difficult to achieve good registration between the two adhesive binder layers on opposite sides of the current collector. These difficulties often necessitate the use of relatively expensive fabrication equipment.
A need thus exists for methods that facilitate registration of the various electrode elements during fabrication of electrodes with relatively low equivalent series resistance. Another need exists for electrodes fabricated using these methods. A further need exists for energy storage devices using such electrodes.